This document summarizes several MAC protocols for wireless sensor networks. It begins by introducing the need for MAC protocols to control medium access in wireless networks and common sources of energy waste. It then categorizes MAC protocols as contention-based (like ALOHA and CSMA), schedule-based (like TDMA), or hybrid. Specific protocols discussed include S-MAC, B-MAC, TRAMA, and hybrid protocols like Z-MAC and Funneling-MAC. The document emphasizes energy efficiency as the primary concern for MAC protocols in wireless sensor networks.

1. 這份投影⽚片是 2007 年製作的，雖
然內容有點舊，但是很多觀念是相
通的，由於沒時間準備公司的「技
術分享會議」只好拿舊東西來說了
說明

2. 2
Medium Access Control
(MAC) Protocols for
Wireless Sensor Networks
!
!
Presented by Chih-Yu

3. 3
Outline
● Introduction
● MAC Protocols
● Contention-based
● Schedule-based
● Hybrid
● Conclusion

4. 4
Outline
● Introduction
● MAC Protocols
● Contention-based
● Schedule-based
● Hybrid
● Conclusion

5. 5
Introduction
● Wireless Sensor Networks
⽕火災、⼟土⽯石流

6. 6
Introduction
● Why do we need MAC?
● Wireless channel is a shared medium
● Radios transmitting in the same frequency band
interfere with each other
!
● The role of Medium Access Control
● Controls when and how each node can transmit in
the wireless channel
● Solves the contention and collision

7. 7
Introduction
● MAC Protocols for WSNs
● Primary Concern
● Energy Efficiency
● Reduction of significance
● Contention/Collision (basic task)
● Scalability
● Latency
● Communication Patterns
● Adaptability to changes
● Fairness
● Throughput

8. 8
Introduction
● Major Sources of Energy Wastes
● Control Packet Overhead
● E.g., RTS/CTS
● Collision
● Retransmission
● Overhearing
● The receiver of a packet is not the intended receiver of
that packet
● Idle Listing
● Listening to possible traffic that is not sent
ab

9. 9
Outline
● Introduction
● MAC Protocols
● Contention-based
● Schedule-based
● Hybrid
● Conclusion

10. 10
Classification of MAC
Protocols
● Contention-based protocols
● Nodes compete in probabilistic coordination
● Examples: ALOHA (pure & slotted), CSMA
● Simple, no time synch, and robust to network
changes
● High idle listening and overhearing overheads
● Solve this by duty cycling

11. 11
Classification of MAC
Protocols
● Schedule-based protocols
● Schedule nodes onto different sub-channels
● Examples: TDMA, FDMA, CDMA
● Collision-Free
● Requires time synch and not robust to changes.
● Low throughput and high latency even during low
contention.
● Low idle listening and overhearing overheads
● Wake up and listen only during its neighbor
transmission

12. 12
Outline
● Introduction
● MAC Protocols
● Contention-based
● Schedule-based
● Hybrid
● Conclusion

13. 13
Contention-based MAC
Protocols for WSNs
● S-MAC
● B-MAC

14. 14
S-MAC
W. Ye, J. Heidemann, and D. Estrin, “An Energy-efficient MAC Protocol
for Wireless Sensor Networks,” in Proc. of IEEE Infocom, Jun. 2002, pp.
1567–1576.
● Basic Idea
● Trades energy efficiency for lower throughput and
higher latency
● Main Components
● Periodic Listen and Sleep
● Collision Avoidance
● Overhearing Avoidance
● Message Passing

15. 15
Periodic Listen and Sleep
● Nodes periodically sleep
● Turn off radio when sleeping
● Reduce duty cycle to ~10% (150ms on vs. 1.5s off)
● Trades energy efficiency for lower throughput
and higher latency
Listen Sleep tSleepListen
Listen Sleep Listen t
Both the synchronization issue and the schedule maintenance issue
can be found in the paper.

16. 16
Collision Avoidance
● Similar to IEEE 802.11
● Virtual Carrier Sense
● NAV (Network Allocation Vector)
● Physical Carrier Sense
● Four-way handshake
● RTS/CTS/DATA/ACK

17. 17
Overhearing Avoidance
● Basic Idea
● A node can go to sleep whenever its neighbor is
talking with another node
● Who should sleep?
● The immediate neighbors of sender and receiver
● How to they know when to sleep?
● By overhearing RTS or CTS
● Hog long should they sleep?
● NAV

18. 18
Message Passing
● How to transmit a long message?
● Transmit it as a single long packet
● Easy to be corrupted
● Transmit as many independent packets
● Higher Control Overhead & Longer Delay
● Divide into fragments, but transmit all in burst
!
!
!
● RTS/CTS reserve medium for the entire sequence
RTS
CTS
Data
CTS
Data
CTS
Data
CTS

19. 19
B-MAC (Overview)
● B-MAC is implemented in TinyOS
!
● Major Feature: reconfigurable
● Above B-MAC, one can implement an RTS-CTS
scheme or a TDMS-like scheduling protocol
!
● A small core of media access functionality
● arbitration, reliability, low power communication
J. Polastre, J. Hill, and D. Culler, “Versatile low power media access for
wireless sensor networks,” in SenSys, Nov. 2004, pp. 95–107.

20. 20
B-MAC (Overview)
● Channel Arbitration
● Clear Channel Assessment (CCA) & Backoffs
!
● Reliability
● Link-layer acknowledgment
!
● Low Power Communication
● Low Power Listening (LPL)

21. 21
Clear Channel Assessment
● This paper proposes a way to estimate noise
floor using signal-processing techniques
● Noise floor can be viewed as a threshold
!
● Where a node intend to transmit a packet
● 802.15.4: takes a single sample and compares it to
the noise floor
● B-MAC: outlier detection algorithm

22. 22
Clear Channel Assessment

23. 23
Clear Channel Assessment
● MacControl
● command result_t EnableCCA();
● command result_t DisableCCA();
!
● By disabling CCA, a scheduling protocol may be
implemented above B-MAC
!
● If CCA is enabled
● event uint16_t initialBackoff(void* msg);
● event uint16_t congestionBackoff(void* msg);

24. 24
Link-layer Acknowledgment
● Similar to traditional MAC protocols
!
● MacControl
● command result_t EnableAck();
● command result_t DisableAck();
!
● RTS/CTS/DATA/ACK ?

25. 25
Low Power Listening
● Low Power Listening
● Sender (Transmit mode)
● Send preamble before transmit the data
● Preamble Length
!
● Receiver (Listening mode)
● Check Interval
!
● To reliably receive data, the preamble length is matched to
the interval that the channel is checked for activity
● If the channel is checked every 100 ms, the preamble must
be at least 100 ms long

26. 26
Low Power Listening
Sender
Receiver 1
Receiver 2
Backoff
& CCA
Preamble
wakeup
Data
wakeup
Check Interval

27. 27
Outline
● Introduction
● MAC Protocols
● Contention-based
● Schedule-based
● Hybrid
● Conclusion

28. 28
Schedule-based MAC
Protocols for WSNs
● TDMA MAC Protocols
● TRAMA
● Convergecast in Tree-based Wireless Sensor
Networks

29. 29
Traditional TDMA MAC
Protocols
● Divide time into cycles
● A cycle consists of several slots
!
!
!
● Advantages
● Collision-Free, Low Idle Listing and Overhearing Overheads
● Disadvantages
● Synchronization, Low Channel Utilization
!
● An important Issue
● Slot Assignment Strategy
1 2 3 4 5 6 7 1 2 3
Cycle
3 4 5 cc 1 2 3 4 5 6 7
Cycle

30. 30
Slot Assignment Strategy
● Basic Slot Assignment Strategy
● A node should own a slot different from its one-
hop and two-hop neighbors
R
S
S
Collision
One-hop
R
R
S S
Collision
Collision
Two-hop
RS R S
Three-hop
Acknowledgement?

31. 31
TRAMA
● TRAMA (TRaffic-Adaptive Medium Access)
● Avoid assigning slots to nodes with no traffic
● Allow nodes to determine when they can switch off
!
● Main Components
● Neighbor Protocol
● Schedule Exchange Protocol
● Adaptive Election Algorithm
V. Rajendran, K. Obraczka, and J. Garcia-Luna-Aceves, “Energyefficient,
collision-free medium access control for wireless sensor networks,”
Wireless Networks, vol. 12, no. 1, pp. 63–78, Feb. 2006.

32. 32
Neighbor Protocol
To accommodate topology dynamics:
1. Dynamic: interval between random access periods could be larger
2. Static: interval could be smaller

33. 33
Schedule Exchange Protocol
Random
Access
SCHEDULE_INTERVAL
SCHEDULE_INTERVAL
Winning Slot
(Intended Receiver: a, b, c)
Winning Slot
(Intended Receiver: d, e, f)
Winning Slot
(Intended Receiver: φ)

34. 34
Adaptive Election Algorithm
x
a y
b 10080
60
20
30
50
10 120
90
130

35. 35
Convergecast in Tree-based
Wireless Sensor Networks
6
4
5
4
1 2 3 1
0 1 0
3
MAXSLOT=7
2
Level 0
L1
L2
L3
1. Construct a BFS tree T
!
2. Traverse the nodes of T in a
bottom-up manner
a) if the node v is a leaf node,
assign a min slot s(v) which is
not interference with its
interference neighbors
b) if v is the in-tree node, choose
the min slot s(v) which is larger
than the children’s slot and do
not interference with others
!
3. Re-assign the slot in a top-down
mannerY.-C. Tseng and M.-S. Pan, “Quick
Convergecast in ZigBee/IEEE 802.15.4
Tree-Based Wireless Sensor Networks”,
ACM MobiWac, 2006.

36. 36
Outline
● Introduction
● MAC Protocols
● Contention-based
● Schedule-based
● Hybrid
● Conclusion

37. 37
Hybrid TDMA/CSMA
● Z-MAC
● Funneling-MAC

38. 38
Z-MAC
● Z-MAC (Zebra MAC)
● A hybrid TDMA and CSMA MAC scheme
● The main feature is its adaptability to the level of
contention in the network
● under low contention, it behaves like CSMA, and
● under high contention, like TDMA
I. Rhee, A. Warrier, M. Aia, and J. Min, “Z-MAC: a hybrid MAC for
wireless sensor networks,” in SenSys, Nov. 2005, pp. 90–101.

39. 39
Z-MAC
● Design of Z-MAC
● Neighbor Discovery and Slot Assignment
● Local Framing
● Transmission Control of ZMAC
● Explicit Contention Notification
● Receiving Schedule of ZMAC
● Local Time Synchronization

40. 40
Funneling-MAC
● Funneling effect in sensor networks
G.-S. Ahn, E. Miluzzo, A. T. Campbell, S. G. Hong, and F. Cuomo “Funneling-
MAC: A localized, sink-oriented MAC for boosting fidelity in sensor networks,” in
SenSys ’06

41. 41
Funneling-MAC
● The authors propose a localized, sink-
oriented funneling-MAC
!
● Localized
● Using local TDMA scheduling in the intensity
region only
● Sink-oriented
● Manage TDMA scheduling
● Compute and maintain the depth of the intensity
region

42. 42
Conclusion
● MAC Protocols in WSNs
● Energy Efficiency and Other Concerns
● Contention-based/Schedule-based/Hybrid
!
● Future Directions
● Taking the traffic features of WSNs into
consideration
● Communication Patterns: Convergecast, Broadcast,
Local Gossip, Event Reporting, etc.